1. Field of the Invention
The present invention relates to a method of determining the position of a catheter in a patient's esophagus, a control unit for controlling the monitoring of an electric signal from a catheter in a patient's esophagus, and a computer program product for use in such a control unit.
2. Description of the Prior Art
It is known in the art to use myoelectrical signals from a patient to control the function of a ventilator providing breathing support to the patient. U.S. Pat. Nos. 5,820,560 and 6,588,423 both disclose methods and devices for triggering ventilatory support to a patient using a myoelectrical signal obtained from the diaphragm. U.S. Pat. No. 5,671,752 discloses a method and a device for registering the myoelectrical activity of the diaphragm by means of an esophageal catheter having an array of electrodes. The signals from such a catheter can be used as the myoelectrical signal to control ventilator function. EP 1 091 780 discloses the use of a neuroelectrical signal picked up, for example, from the phrenic nerve to control a ventilator.
A problem when obtaining a myoelectrical signal from the diaphragm is positioning of the catheter within the patient's esophagus. To obtain a proper signal some of the electrodes of the catheter should be placed above the diaphragm and some below it. There is a possibility that the catheter will be inserted too far, or not be inserted far enough. In both cases, the catheter will detect a weak signal, or may not capture any signal at all. Alternatively, the catheter may capture myoelectrical signals from other muscles instead of, or in addition to the signal from the diaphragm. Hence, it is difficult to obtain an optimal catheter position and the ventilator may have to work in pneumatic triggering mode if the signal is too weak.
Co-pending Swedish Application No. SE 0702191-8 discloses a method of positioning the catheter based on the ECG component that will always be present in a myoelectrical signal from the diaphragm. In this application, the damping of the ECG signal caused by the diaphragm is used. The ECG signal components from different electrode pairs are determined and compared and the difference in amplitude of the ECG signal between different electrode pairs is used to determine the position of the diaphragm relative to the electrode pairs. The greatest damping between two neighboring electrode pairs should be caused by the diaphragm being positioned between these two electrode pairs. This method is predominantly based on the registration and comparison of the QRS complex of the ECG signal.